The US air force has a long history of friendly fire incidents, in part because it is the world's overwhelmingly dominant air power, but concerns have been raised over the training of reservist pilots, amphetamine use and the reliance on outdated technology to distinguish friend from foe.

The two US pilots involved in the strafing of the Household Cavalry regiment near Basra in 2003, whose voices were recorded on the cockpit videotape, were both reservists with the Idaho Air National Guard. In another friendly fire incident, which killed four Canadians in Afghanistan the previous year, the two pilots were also National Guard part-timers.

Sam Gardiner, a retired US air force colonel who has taught military strategy at the Air War College, said National Guard pilots do not have as much experience as regular air force pilots in combined exercises with Nato allies. As a result they are not used to seeing British vehicles below them.

"These guys are trained well enough to fly the airplanes and do the missions, but when it comes to complex multinational missions, they have not had that Nato experience," Mr Gardiner said.

That claim was denied by Lt Col Catherine Reardon, a US air force spokeswoman, who said there was no difference in the capabilities of reservist, National Guard and regular air force pilots. "Pilot training is pilot training," she said. She did not have statistics showing whether or not reservist and National Guard pilots were involved in a disproportionate number of friendly fire incidents.

What appears to be beyond dispute is the lack of progress in developing a modern method of definitively identifying allied forces on the ground, to avoid lethal mistakes. From the cockpit transcript, it is clear the pilots see the orange panels on the top of the British vehicles but ultimately decide they are rocket launchers.

"This goes back to the invasion of Europe in 1944. On the front of our vehicles we had orange panels," Mr Gardiner said. "They are still using 60-year-old technology."

Another unknown is the role of drugs. Lawyers for the two National Guard pilots court martialled for the Canadian deaths in Afghanistan raised the possibility that their actions could have been influenced by dextroamphetamine "go pills" routinely taken by US combat pilots to keep them alert on long sorties. Side-effects include agitation, paranoia and psychosis. British pilots are banned from taking such stimulants. They were banned for five years by the US air force, but reinstated in 1996.

It is not clear what issues were raised in a series of internal US air force inquiries into the incident that killed Lance Corporal Matty Hull, because the deliberations have not been made public. But it is almost certain the pilots involved will not appear in a British court. The US does not extradite its fighters, and fought the creation of the international criminal court largely on that principle.